When electronically processing a document such as, for example, a survey form or the like having respondent information entered thereon, it is often desirable to remove or dropout the background of the document from a scanned image of the document. Doing so isolates the respondent information from the background of the form thereby facilitating storage of the image and subsequent processing of the respondent information therein. Removal of color backgrounds from a color form is often done using bitonal scanners. Bitonal scanners use a particular color of bulb, thereby effectuating removal of particular colors of ink from the scanned images of the documents. For example, red bulbs are typically used to remove form backgrounds which are composed of a red hue. Forms are designed to take advantage of this by creating backgrounds that predominantly consist of a single of color ink (e.g., a dropout color) so that the background is dropped from the form when it scanned using a bitonal scanner with the proper color bulb. Unfortunately the bitonal scanner also removes any respondent information that is written in a color similar to the dropout color (e.g., in red pen).
Another technique applied to remove backgrounds from bitonal images of scanned documents is pixel-by-pixel comparison of the bitonal image with a template image. After the bitonal image is obtained from the bitonal scanner it is registered against a bitonal template. The registered bitonal image is compared pixel-by-pixel against a dilated version of the bitonal template. Determining which pixels to keep for a bitonal form dropout is much easier because there are only two possible values for each pixel, namely black and white. Only those pixels where the respondent pixel is black and the dilated template image is white are kept in the dropout image.
An alternative to bitonal scanning is color scanning. Color scanning is advantageous over bitonal scanning because it does not eliminate colors from the resulting image. However, the foregoing pixel-by-pixel comparison techniques used for bitonal images are ineffective on color images because each of the pixels can be a multitude of different colors. One common method of addressing this issue for color images is to select particular colors on the form to dropout of the entire form. All colors on the entire form that are the same as the particular colors are removed, but if the respondent wrote in a color that is the same or similar enough to one of the dropout colors, the respondent data is removed as well.